OWL: The Web Ontology Language

PowerPoint-PräsentationSemantic Web Lecture
www.sti-innsbruck.at
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Extensions
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First, why RDF is not enough and we need other representation formalism?
This formalism is OWL an extension of RDFS
www.sti-innsbruck.at
RDF Schema
RDFS Vocabulary
RDFS Metadata
Semantics of RDF and RDF Schema
Semantic notions
RDF(S) Entailment
RDF Schema
RDFS Vocabulary
RDFS Metadata
Semantics of RDF and RDF Schema
Semantic notions
RDF(S) Entailment
RDF Schema
RDFS Vocabulary
RDFS Metadata
Semantics of RDF and RDF Schema
Semantic notions
RDF(S) Entailment
RDF Schema
RDFS Vocabulary
RDFS Metadata
Semantics of RDF and RDF Schema
Semantic notions
RDF(S) Entailment
RDF Schema
RDFS Vocabulary
RDFS Metadata
Semantics of RDF and RDF Schema
Semantic notions
RDF(S) Entailment
RDF Schema
RDFS Vocabulary
RDFS Metadata
Semantics of RDF and RDF Schema
Semantic notions
RDF(S) Entailment
RDF Schema
RDFS Vocabulary
RDFS Metadata
Semantics of RDF and RDF Schema
Semantic notions
RDF(S) Entailment
Containers
Collections
Reification
Domain and range of property infer information rather than check data
Conjunctive interpretation of multiple restrictions
Use of properties as objects
RDF/XML syntax very verbose
Classes
Properties
e.g. for class Person, the property hasName has range xsd:string
Complex concept descriptions
Cardinality restrictions
Disjointness axioms
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Verbose Syntax
Every graph is valid
e.g. <rdfs:Class, rdfs:subClassOf, ex:a>
RDF
RDF Schema
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This and following slides in part due to Frank van Harmelen http://www.cs.vu.nl/~frankh/spool/SemWebSlides/OWL.ppt
www.sti-innsbruck.at
Extending RDF Schema
OWL extends RDF Schema to a full-fledged knowledge representation language for the Web
logical expressions (and, or, not)
(in)equality
Ontologies are extendable
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From Grigoris Antoniou and Frank van Harmelen: A Semantic Web Primer, MIT Press 2004
www.sti-innsbruck.at
Ontologies are extendable
Equality (for classes, for individuals)
Classes as instances
Disregarded:
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A logical system or theory is decidable if there exists an
effective method such that for every formula in the system
the method is capable of deciding whether the formula is valid
in the system or not.
OWL Full
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OWL Full
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but cannot be solved in practice are called intractable.
OWL Lite
(sub)classes, individuals
OWL Full
No restriction on use of vocabulary (as long as legal RDF)
Classes as instances (and much more)
RDF style model theory
Reasoning using FOL engine
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Use of vocabulary restricted
Can’t be used to do “nasty things” (e.g. modify OWL)
No classes as instances
Defined by abstract syntax
Standard DL-based model theory
Reasoning for full language via FOL engines
No need for axiomatization
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Restricted cardinality (0/1)
No nominals (oneOf)
Semantically, only small restriction on OWL DL
No nominals
Named classes (A)
Named properties (P)
OWL DL corresponds to the DL SHOIN(D)
Property value (∃ P.{o})
∃
OWL Lite has strong syntactic restrictions, but only limites semantics restrictions, compared with OWL DL
Negation can be encoded using disjointness
With negation and conjunction, you can encode disjunction
Class(C complete unionOf(B C))
More on layering
For an OWL DL-restricted KB, OWL Full semantics is not equivalent to OWL DL semantics
John friend Susan
OWL Full entails:
John rdf:type owl:Thing
Susan rdf:type owl:Thing
friend rdf:type owl:ObjectProperty
John rdf:type _:x
_:x owl:onProperty friend
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UML
Abstract syntax
Human readable
...
xmlns:food=”http://www.w3.org/TR/2004/REC−owl−guide−20040210/food#”
... >
...
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xmlns:food=”http://www.w3.org/TR/2004/REC−owl−guide−20040210/food#”
... >
...
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From Grigoris Antoniou and Frank van Harmelen: A Semantic Web Primer, MIT Press 2004
www.sti-innsbruck.at
≤1isTaughtBy
Class(course partial restriction (isTaughtBy minCardinality (1)))
Class(department partial restriction (hasMember minCardinality(10))
restriction (hasMember maxCardinality(30)))
An example of usage of OWL
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An Ontology describing wine domain
One of the most widely used examples for OWL and referenced by W3C.
There is also a wine agent associated to this ontology that performs OWL queries using a web-based ontological mark-up language. That is, by combining a logical reasoner with an OWL ontology.
The agent's operation can be described in three parts: consulting the ontology, performing queries and outputting results.
Available here: http://www.w3.org/TR/owl-guide/
The practical experience in building applications has revealed several shortcomings of OWL, and hence Description Logics for modeling knowledge
Feature that ontology engineers are missing:
Higher Relational Expressivity
Polyadic Predicates
Close-World Reasoning
Integrity Constraints
Modeling Exceptions
Proper combination of Description Logic and Horn Logic can define extensions of the language to support such features
OWL-Flight
Higher Relational Expressivity.
While OWL allows to describe concepts through a rich set of primitives;
its capability to express knowledge about roles is very reduced,
and do not corresponds to the sets of primitives required by real world
applications.
Polyadic Predicates.
OWL support only unary and binary predicates. While in many real world application, n-ari predicate are required
3. Close-World Reasoning.
OWL and DLs are based on the Open-World Assumption (OWA): i.e. what is not currently known to be true is undecided
(neither true neither false). This assumption is often colliding with the needs of applications
for which the Closed-World Assumption (i.e. what is not currently known to be
true is false) is more appropriate.
4. Integrity Constraints.
Integrity constraints cannot be easily expressed in OWL.
This is due the fact that OWL adopts the Open-World Assumption and does not
adopt the Unique Names Assumption (UNA) (i.e. two resources with different
identifiers are different resources).
5. Modeling Exceptions.
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(or summary)
Jos de Bruijn: Using Ontologies. Enabling Knowledge Sharing and Reuse on the Semantic Web. DERI Technical Report DERI-2003-10-29, 2003.
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